Handle user-set pgBouncer logfile for OTEL, mkdir

[benjaminjb]

pgBouncer allows users to set the log destination; when OTEL is enabled, we assume that destination is the default that we set. This PR patches that hole by setting OTEL to use the either the default or the user-set log destination.

This PR also handles creating the destination dir for the logfile.

This is a preliminary step on the way to validating the log config in v1.

Checklist:

• Have you added an explanation of what your changes do and why you'd like them to be included?
• Have you updated or added documentation for the change, as applicable?
• Have you tested your changes on all related environments with successful results, as applicable?
  • Have you added automated tests?

Type of Changes:

• New feature
• Bug fix
• Documentation
• Testing enhancement
• Other

What is the current behavior (link to any open issues here)?

Our pgBouncer OTEL settings assume a user hasn't set the logfile config, and won't work right if they do.

What is the new behavior (if this is a feature change)?

• Breaking change (fix or feature that would cause existing functionality to change)

This updates OTEL with what the user sets.

Other Information:
Issues: [PGO-2565]

[dsessler7]

There is some overlap between this PR and #4252

[benjaminjb]

There is some overlap between this PR and #4252

Yeah, I wanted to break down that PR into discrete parts so we could focus on the things that we still have questions about, e.g., CEL validation rules + costs, using the Log config struct

[dsessler7]

LGTM